I received my 70-300L last week and took it to the zoo over the weekend. It was a real treat. It felt faster than it was, though on occasion I'd struggle to get a usable shot of an animal in the shade. I got several sharp shots at 1/20s and 300mm (though to be fair, it was probably about a 50% usable rate at that speed and focal length). The autofocus was surprisingly fast on my 5D2. I was afraid that, being a slower lens, it would suffer. Perhaps best of all, carrying it around with me all day did not get tiring or cumbersome.

Overall, I'm very happy with it. It has a reach advantage over the 70-200 lenses, a weight advantage over the 70-200 f/2.8, and is just barely slower than the 70-200 f/4. For my needs, the extra focal length and smaller size suits me perfectly. The build quality is fantastic; it feels a step above my 100L macro or 24L II. The bokeh is great, and it's still possible to get fairly isolating DoF wide open and at 300mm if you watch for good backgrounds.

Superb sharpness and contrast, beautiful bokeh, great size and weight, IS is very effective

Cons:

Expensive for as little an improvement as there is over the non-L, noticeable vignetting wide open, some very slight longitudinal CA wide open

I use this lens on a 5D Mark II. It is phenomenally sharp even into the corners, and even wide open. Stopping down is entirely optional for sharpness. Unfortunately there is some significant vignetting when wide open, which is what kept me from giving the lens a 10. It's almost perfect in IQ, but not quite. I also think it would make more sense for it to be priced closer to the non-L.

It's of course great for macro shots, but it's also a wonderful walkaround lens (for FF at least). Portraits are definitely a strength of this lens. At f/2.8, which this lens does so well, background details blur away pretty well, and the bokeh is great.

Focusing moves very fast and is accurate. Only downside is that without the limiting switch engaged, it can hunt through a pretty big range before locking.